1. Field of the Invention
The present invention relates to a discharge lamp driving circuit for lighting a discharge lamp to illuminate a liquid crystal display (LCD) apparatus, and more specifically to a discharge lamp driving circuit provided with a function of detecting electric discharges, such as an arc discharge, and a corona discharge, that occur in the circuit.
2. Description of the Related Art
A backlight system has been generally used as a lighting device for an LCD apparatus. The backlight system comprises: one or more discharge lamp, such as a cold cathode lamp and a metal halide lamp; and a discharge lamp driving circuit composed of various circuits, such as an inverter circuit to drive the discharge lamp. Since a high voltage is required for lighting the discharge lamp, the discharge lamp driving circuit has a high-voltage transformer, and the discharge lamp is connected to the secondary side of the high-voltage transformer. The discharge lamp driving circuit is equipped with a protector by which the circuit is deactivated, for example, when a lamp current flowing in the discharge lamp has an abnormally high value. In such a discharge lamp driving circuit, when a lamp current flowing in the discharge lamp exceeds a predetermined value and it is duly detected, a protection circuit is activated thereby preventing an overcurrent from flowing in the discharge lamp (refer to, for example, Japanese Patent Application Laid-Open No. 2003-168585).
FIG. 1 is a block diagram showing a discharge lamp driving circuit 50 disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2003-168585. In the circuit 50, the primary side of a transformer 51 is connected to an H-bridge circuit 52 which drives the primary side of the transformer 51, and to which a logic circuit 53 to produce a signal to be sent to the H-bridge 52 is connected, while the secondary side of the transformer 51 is connected to a discharge lamp 54, and also to a protection circuit 55 which, when a voltage of a signal 56 at the secondary side of the transformer 51 exceeds a predetermined value, deactivates the logic circuit 53 thereby preventing an overcurrent from flowing in the discharge lamp 54.
Since s discharge lamp used in a backlight system as a lighting device for an LCD apparatus must be driven by a high voltage, a high-voltage transformer is provided in a discharge lamp driving circuit. So, if a withstand voltage is lowered due to poor connection of a wiring to the secondary terminal of the transformer, breakage of a wiring at the secondary side of the transformer, poor connection between connector terminals for the discharge lamp, defective wires of the discharge lamp, or poor insulation of coils in the transformer, then an arc discharge, or a corona discharge can occur at some small gaps or voids found at the defective or poor areas. The arc discharge is accompanied by sparks, and may damage terminals and components, or even cause smoking or firing, resulting in possibly damaging the discharge lamp driving circuit and the LCD apparatus. Therefore, in a discharge lamp driving circuit provided with a high-voltage transformer, it is necessary to detect a corona discharge or an arc discharge, and to stop supplying electric power to a discharge lamp upon detection of such discharges thereby preventing damages to the circuit and the LCD apparatus.
In the discharge lamp driving circuit 50 described above, the logic circuit 53 is deactivated when the voltage of the signal 56 at the secondary side of the transformer 51 exceeds a predetermined value, whereby an overcurrent is prevented from flowing in the discharge lamp 54. However, since the discharge lamp 54 is kept on lighting even if a corona discharge or an arc discharge occurs at partially broken wires at the secondary side of the transformer 51, the lamp current is kept at a constant value, and therefore the protection circuit 55 fails to duly function. Thus, the discharge cannot be detected.
It is required that a discharge occurring at partially broken wires at the secondary side of a high-voltage transformer be detected in order to stop supply of electric power for the purpose of protecting the circuit. Under the circumstances, a discharge lamp driving circuit is proposed, in which a corona discharge occurring near a high-voltage transformer or a discharge lamp is detected at its very start for protection of the circuit (refer to, for example, Japanese Patent Application Laid-Open No. 2002-341775). Such a discharge lamp driving circuit includes an induction pattern for a transformer, and another induction pattern for a discharge lamp, and voltages induced in the induction patterns are detected for protection of an inverter circuit.
FIG. 2 is a block diagram showing a discharge lamp driving circuit 60 disclosed in the aforementioned Japanese Patent Application Laid-Open No. 2002-341775. The discharge lamp driving circuit 60 has an induction pattern 62 for a transformer shaped into an elongated rectangle and disposed at an area in the lower face of a printed circuit board corresponding to a high-voltage transformer 61, and an induction pattern 64 for a discharge lamp shaped into a rectangle and disposed at an areas in the lower face of a printed circuit board corresponding a discharge lamp 63. When a corona discharge occurs at the transformer 61 or the discharge lamp 63, voltages induced at the induction patterns 62, 64 become high-frequency currents and are compared with respective reference voltages at corona discharge detecting circuits 65, 66. If the induced voltages are lower than the reference voltages, a signal is outputted so as to deactivate a switching transistor for protection of the circuit 60.
Since the discharge lamp driving circuit as described above is formed on a printed circuit board which is dimensioned substantially as large as a light conductive plate disposed on the bottom face of a liquid crystal cell, the printed circuit board has to be inevitably dimensioned as large as the liquid crystal cell. Consequently, in a large LCD apparatus used in, for example, a large TV, a large printed circuit board must be used in accordance with the size of the liquid crystal cell, which results in an increased cost of the discharge lamp driving circuit. Also, respective induction patterns must be disposed for a transformer and a discharge lamp thus constituting another factor for an increased cost. Further, since a plurality of transformers and discharge lamps are provided in a large LCD apparatus, typically in a liquid crystal TV, a plurality of induction patterns must be provided for a transformer and a discharge lamp, respectively, which results in requirement of a number of induction patterns, and also which makes the induction pattern arrangement difficult.